This invention relates to exposure indicating devices for use in lens-exchangeable, single lens reflex cameras of the through-the-lens (TTL) light measurement type, which is applicable to any type of objective lens selected, and wherein, in any selected mode of automatic exposure control, the exposure factor values to be automatically controlled may be properly indicated prior to the commencement of the diaphragm aperture stopping-down operation.
U.S. Pat. No. 3,829,867 discloses two automatic exposure control modes, namely, the so-called shutter-speed-priority-automatic-diaphragm-control mode, and the so-called diaphragm-priority-automatic-shutter speed-control mode. In the former control mode, the diaphragm is stopped-down from an open position in association with the shutter release operation prior to the commencement of the movement of the shutter to an open position, while the intensity of light which has passed through an objective is measured and the diaphragm is in the process of being stopped-down. When a proper aperture value for a preselected shutter speed is determined as the result of the light measurement, the stopping-down operation of the diaphragm is interrupted. With the diaphragm maintained stopped-down at that aperture value, light measurement for shutter speed control is conducted, so that the shutter speed may be controlled according to the light measuring outputs. In the latter control mode the diaphragm is stopped-down to a preset aperture value in association with the shutter release operation, prior to the commencement of the movement of the shutter to an open position, and the light measurement at the aperture value to which the diaphragm has been stopped-down is conducted, so that the shutter speed is controlled according to the light measuring outputs so obtained.
Various attempts have been proposed for providing a camera capable of changing over the exposure control mode from the diaphragm-priority-automatic-shutter-speed-control mode to the shutter-speed-priority-automatic-diaphragm-control mode, and vice-versa, in a TTL light measuring single lens reflex camera equipped with an exposure indicating device. However, some of these cameras, particularly a camera with an exchangeable lens, fail to transmit sufficient information from the lens to the camera body to achieve a complete indication in each exposure control mode, thereby resulting in imperfect indications of the exposure factors. Stated differently, in the shutter-speed-priority-automatic-diaphragm-control mode, the difficulty arises with such a camera to indicate an f-number corresponding to an aperture value to be automatically controlled prior to the stopping-down operation.
Every exchangeable lens has an inherent minimum f-number. Thus, in order to compute the proper aperture value relative to a brightness value, the film sensitivity setting and a preselected shutter speed, and to indicate an f-number corresponding to the proper aperture value, the intrinsic minimum f-number of the exchangeable lens mounted on the camera, as well as other factors, need to be transmitted to the camera body. Moreover, the manufacturers or suppliers of a new type camera or lens should consider the exchangeability of a new camera or lens with an old type camera or lens which has been in the market for a long period of time to avoid supplying a new type camera body on which an old type lens cannot be mounted, or a new type lens which cannot be mounted on an old type camera.
The indication of an f-number corresponding to an aperture value to be controlled in the shutter-speed-priority-automatic-diaphragm-control mode poses a critical problem for camera manufacturers who have supplied a lens having only a transmitting member for transmitting information from the exchangeable lens to the camera body of the difference in the number of steps to be stopped-down from a minimum f-number to a preset f-number. Moreover, it is common that single lens reflex cameras as described above are provided with a mode selection member on the camera body for selecting either the automatic aperture control mode or the automatic shutter speed control mode.